The present invention relates to television subcarrier to horizontal sync phase circuits, and more particularly to a circuit for quantizing television horizontal phase to subcarrier zero crossings to assure that horizontal sync pulse leading edges occur at the extrapolated zero crossings of the subcarrier burst.
NTSC composite color video signals contain not only picture information (luminance and chrominance), but also timing information (vertical and horizontal sync pulses) and other reference information (equalizing pulses and color burst). The horizontal sync pulse and the color burst occur during the horizontal blanking interval of each scan line. In accordance with the Electronics Industries Association (EIA) standard RS-170A the start of the color burst is defined by the zero crossing (positive or negative slope) that precedes the first half cycle of the subcarrier. The reference subcarrier from the color burst is used in a television receiver to control a phase locked oscillator which generates a continuous wave at subcarrier frequency in order to extract the chrominance information from the composite video signal. In accordance with RS-170A the zero crossing of the extrapolated color burst, i.e., the continuous wave at subcarrier frequency and in phase with the color burst, must be coincident with the sync point, i.e., the fifty percent point on the leading edge of the horizontal sync pulse. Although the slope of the zero point from field to field in the NTSC format changes, the zero point of the extrapolated subcarrier must maintain that coincidence.
What is desired is a circuit for automatically assuring the correct phase relationship between the extrapolated subcarrier frequency zero crossings and horizontal sync.